Duplicate keys in mappings don't cause parse error

[rhysd]

YAML spec says at 3.2.1.1:

The content of a mapping node is an unordered set of key: value node pairs, with the restriction that each of the keys is unique.

So keys in mappings MUST be unique. This is different from JSON spec where keys SHOULD be unique.

To conform the YAML spec, any YAML parser should cause parse error on duplicate keys. However this library does not cause a parse error.

package main

import (
	"github.com/goccy/go-yaml/parser"
)

func main() {
	s := "foo:\nfoo:\n"
	_, err := parser.ParseBytes([]byte(s), 0)
	if err == nil {
		panic("should have caused parse error")
	}
}

Here is a link to playground: https://play.golang.org/p/xog5p3vkQFj

[marco-m]

Hello @goccy , I stumbled on the same problem. For my better understanding of the suitability of go-yaml, could you please give us your idea? That is, do you consider this a bug or expected behavior? Thanks!

[goccy]

@marco-m @rhysd Thank you for your reports. I plan to fix this.

[rhysd]

Checking duplicates affects performance (because the parser needs to remember the previous keys). So I'm not sure this check is needed as the default behavior. In the case, adding some opt-in/opt-out check may be suitable to control the trade off.

[goccy]

I've fixed this behavior with #531 . I decided to make an error as the default behavior.